The present invention relates to antenna structures and in particular to self-erecting whip antenna structures.
Self-erecting antennas are often used in situations where stowage space is limited and where deployment must occur unattended by a human operator. The need to deploy a vertical antenna several feet long from a relatively small unattended electronic device, such as an expendable RF transmitter and receiver, poses special problems. Package size and geometry constraints coupled with operational requirements for such a device typically call for an antenna which has the following characteristics:
1. Self-erecting. PA1 2. Capable of being collapsed and stowed in a relatively small volume. PA1 3. Self-supporting against gravitational loading after erection. PA1 4. Capable of remaining erect and functional when exposed to moderate winds and/or when the device to which it is attached is situated on a non-horizontal surface. PA1 5. Incapable of causing the device to which it is attached to be overturned or displaced during antenna deployment and/or during exposure to moderate wind after erection when the device is situated on either a horizontal or nearly horizontal surface. PA1 6. Low visual profile.
Providing a relatively long antenna with such characteristics for a small electronic device pits the requirements for sufficient strength and elasticity to allow stowage and self-erection against the requirements for section stiffness sufficient to prevent excessive flexure or instability (i.e., buckling). In addition, geometric section properties required to prevent instability of relatively long whip antennas subject to gravitational and wind loading are pitted against geometric section properties required to minimize the overturning moment produced by wind loading (i.e., minimize wind drag area) and produce a low visual profile.
One approach to self-erecting antenna structures entails the use of a strip of metal coiled on a storage reel. When released, the strip uncoils into a length of flat metal suitable for use as an antenna. This type of design suffers from a propensity to buckle when exposed to wind loading or gravitational self-loading at lengths of several feet.
In a variation of the coiled metal strip approach, typified in U.S. Pat. No. 3,144,104, an elongated strip is permanently deformed into a hollow tube. The tube is then temporarily opened flat and coiled on a reel. When the reel is released, the strip uncoils and reforms into a tubular shape.
Another variation, typified in U.S. Pat. No. 3,467,329, involves a helically prestressed strip of metal, formed so that adjacent turns are set to coil tightly in an overlapping and telescoping engagement. The strip of metal is wound into a cylindrical coil for storage. When released, the strip uncoils and reforms into a spirally wound, slightly tapered tubular shape.
In both of these variations, in order to prevent buckling due to wind loading or due to self-loading at lengths of several feet, an increase in the cross section of the tubular antenna structure has been required. In these devices, buckling is a "nonhealing" condition and constitutes practical destruction of the antenna structure. Unfortunately, cross sections that are sufficient to prevent buckling due to wind loading or gravitational self-loading are also enough to cause overturning of relatively small devices to which the antennas may be attached. Relatively small devices are also overturned by wind or self-loading of such antenna structures when the devices are placed on surfaces that are not horizontal, and such devices can even be overturned by the energy released during the erection of the antenna. Furthermore, a large cross section also presents a higher visual profile than is desirable where visual unobtrusiveness is required.
Another approach to preventing flexure has been to use stiffer materials to form the antenna structure. As revealed in French Pat. No. 2,312,864, and U.S. Pat. No. 4,134,120, graphite filaments can be used to increase the stiffness of an antenna. However, the antennas described have bending stiffness properties (i.e., modulus of elasticity and moment-of-inertia) which make them too stiff to be coiled so that they can be conveniently stowed in a relatively small volume and can still subsequently use the strain energy stored during coiling to enable them to be self-erecting. In fact, their strength/elasticity properties and described length/diameter ratios would preclude storage in a coiled form of any reasonable diameter.
In an attempt to solve problems of bending due to thermal stress such as found in a space environment it has been suggested the coiled strip and graphite composite approaches could be combined. U.S. Pat. No. 3,975,581 discusses the use of composite prestressed tapes constructed to unfurl into a strip or tube for use as a boom or as a support for coaxial transmission lines. Long structures constructed according to this suggestion would tend to overturn relatively small devices to which they were attached if use were attempted in an earth environment and would not possess an optimally low visual profile.
All of the approaches discussed above require a greater volume for stowage than is desirable for use in relatively small devices. None of the designs could meet the simultaneous constraints of elasticity, stiffness, low mass, elastic stability, low visual profile and low aerodynamic drag area imposed on self-erecting antennas for use with relatively small devices in an earth environment.